The present invention relates to films made from blends of thermoplastic polymer materials which have improved properties. More particularly, the present invention relates to films made from blends of a propylene polymer, a butylene polymer with a low melt index and a butylene polymer with a high melt index which are heat shrinkable and which have good clarity and good processability.
Thermoplastic blends for films are used as packaging material, and in the area of shrink packaging, for objects that are packaged in thermoplastic shrink film. Shrink film is used in many applications, for example, for many types of packaging and wrapping articles such as toys, sporting goods, stationary, greeting cards, hardware, household products, office supplies and forms, phonograph records, industrial parts, computer floppy diskettes, and photo albums, etc. Heat is applied to the film and the film shrinks to conform to the shape of the article packaged therein.
Many thermoplastic films shrink to some extent if they are subjected to elevated temperatures. Use is made of this characteristic by subjecting objects packaged in such films for a short time to elevated temperatures, e.g. exposing them to a blast of heated air, or by immersing in boiling water so that the film shrinks, thereby tightly enclosing the objects packaged therein. Examples are films fabricated from polyolefins or irradiated polyolefins.
Usually for most shrink film applications, a film should exhibit a high shrink energy or contractile force when exposed to elevated temperatures. In addition, the film should not only be heat shrinkable but have good clarity and be easily processed.
A shrink film should possess the following specific properties:
(1) the shrink force should be between 100 and 400 grams per inch at 100.degree. C. depending on the objects to be encased.
(2) the percent shrinkage should be between 10 and 50% at 121.degree. C. depending on the objects to be encased.
(3) the film should have high clarity.
(4) the modulus should be between 60,000 and 350,000 psi depending upon the objects to been encased.
(5) machinability: the coefficient of friction should be less than 0.5.
(6) tear strength: the tear strength should be as high as possible; typical is 3 to 15 grams per mil of film thickness and per inch of width.
(7) elongation: the elongation should be between 50 and 150% depending on the objects to be encased.
Films may be oriented or unoriented. Oriented films may be obtained by stretching processes in which tensions capable of stretching the film are applied to the film, the directions of which form an angle of about 90.degree. utilizing well known prior art techniques. These film stretching tensions may be applied sequentially, as in the case of the film, after forming, is subjected to stretching in a longitudinal direction and thereafter tension is applied in a transverse direction to stretch the film transversely, or simultaneously, whereby longitudinal and transverse tensions are applied to the film at the same time resulting in a simultaneous longitudinal and transverse stretching of the film.
U.S. Pat. No. 3,900,534 discloses a biaxially oriented thermoplastic film structure formed from a blend comprising polypropylene and polybutene homopolymers where the polybutene is present in a small amount of more than 10% but less than 20% by weight.
U.S. Pat. No. 3,634,553 discloses a heat shrinkable oriented thermoplastic film which comprises a blend of polypropylene and an ethylene/butene-1 copolymer.
European Patent Applications 145,014A discloses a blend of a random copolymer of propylene and an alpha olefin with 4 or more carbon atoms (i.e. perhaps butene-1), where the alpha olefin content in the copolymer is 8 to 30 mole % (m %).
Single layer shrink films based on blends of polybutylene with polypropylene are disclosed in Mobil Patents: U.S. Pat. No. 3,634,552 (1972), U.S. Pat. No. 3,364,553 (1972), U.S. Pat. No. 3,849,520 (1974) and U.S. Pat. No. 3,900,534 (1975) and blends of polybutylene with ethylene vinyl acetate (EVA) and C.sub.2 -C.sub..alpha. elastomer or polybutylene with low density polyethylene (LDPE) and C.sub.2 -C.sub..alpha. elastomer (where C.sub..alpha. is an .alpha.-olefin comonomer) are disclosed in U.S. Pat. No. 4,379,888 (1983). Multilayers may include three layers (propylene-ethylene plus butene-1-ethylene plus ethylene-propylene rubber)/tie layer/linear low density polyethylene (LLDPE) by Union Carbide, U.S. Pat. No. 4,196,240 (1980) for frozen poultry and U.S. Pat. No. 4,207,363 (1980) for primal meat cuts. Three layers of propylene-ethylene/(EVA+butene-1-ethylene)/propylene-ethylene, U.S. Pat. No. 4,194,039 (1980) is known. Also three layers (polyproylene+polybutylene)/EVA/irradiated EVA by Cryovac, U.S. Pat. No. 3,754,063 (1973), U.S. Pat. No. 3,932,274 (1974 ), and U.S. Pat. No. 3891,008 (1975) for turkey bags are known.
Polyvinyl chloride (PVC) has been used to produce good shrink films. PVC has been shown to be better in certain applications than the use of polyolefins such as propylene polymers. The use of polyolefins in shrink wrap results in a moderate to high shrink force which is undesirable in many applications. However, the use of polyolefins allows the use of high speed automated packaging machinery with lower cost, and less corrosion of equipment. PVC, however, may produce a better looking package because of the low shrink force and better optics. Also, the seal and shrink of PVC films may take place over a much broader temperature range and tear strength may be better.
There has been a need for a blend of heat shrinkable thermoplastic film with the film advantages of PVC but which is of low cost, can be used on a high speed automated packaging machine and which does not corrode equipment. The polybutylene polypropylene polymer blend film of the present invention exhibits low shrink force which is adjustable by the blending ratio, low shrink temperature, low stiffness, better optics, low corrosion, low cost, and can be used on high speed automated packaging machines.
It has been found that properties can be improved and processing difficulty can be alleviated by adding a small amount (less than 10 w%) of high melt flow or low molecular weight polybutylene as a third component to the PP-BB blends. The new film composition may reduce the scraps generated at the stretching operation during the fabrication of biaxially oriented low shrink force shrink films.
Improvement of the processing characteristics in the fabrication of plastic products has long been demanded due to economical reasons. Optical and shrink properties are considered as critical as the processing characteristics especially in the shrink film packaging industry. Frequently, additives used to improved processability have detrimental effects on physical and optical properties of the products. The addition of high melt index polybutylene into the polypropylenepolybutylene blend of this invention unexpectedly improves both processability and properties.